As a cat kept as a pet becomes older, it becomes more subject to kidney diseases. Since the diseases occupy a higher rank of the cause of death of cats, early diagnosis for kidney diseases in cats has become an important object in the clinical settings of the veterinary clinical field for small animals. A measurement of a protein quantity excreted in urine has been used as an initial diagnosis for kidney diseases in humans. Proteinuria has been utilized as a diagnostic marker to indicate a renal abnormality at an early stage. However, in cats, physiological proteinuria (cauxin urine) and diseased proteinuria cannot be distinguished with a commercially available urine test strip because a high level of proteinaceous cauxin derived from the kidney of even healthy cats exists in the urine. A method for detecting kidney diseases in cats by measuring urinary cauxin has been reported, but the method requires a special reagent (cf. Patent Document 1). Therefore, a test strip for protein measurement generally used in the urinalysis cannot be used, and development of a simple and easy analysis method is desired. In addition, since there is a big sex difference in the content of cauxin present in the urine, it has been difficult in this respect to detect kidney diseases by the detection of cauxin.    Patent Document 1: Japanese Patent Application Laid-Open No. 2003-250575